In using implantable medical devices for recording ECG's or other physiologic data, the data available when the record is made can be useful in interpreting the saved signal records. Various things like the fact of occurrence or nature of the automatic trigger that activated storage of data, and the noise present when the record is made can be invaluable in sorting out the data record to eliminate false indications of medical conditions and discover actual problems which would otherwise remain hidden in the data or lost forever. Triggers and noise should be available as information in recorded electrograms in memories of implantable medical devices. Particularly where the recording of electrogram data is done in the far field, there will be considerable noise and the interpretation of ECG's reproduced from such recorded data will benefit from the storing of information regarding contemporaneous noise. Thus we believe it will be extremely beneficial to the practice of medicine and to medical research if contemporaneous trigger data and noise data could be stored directly in the ECG data, which could them be played back through an external display system with minimal loss of ECG data. The confusion of data by physiologic signals other than the ECG and with the trigger information can be problematic. It becomes even more problematic and detrimental to the data record in the context of desires to store implantable medical device system data, and to achieve a good reproduction of actual physiologic signal data at the same time, with complexities arising from compression needs and limited bandwidth and time to maintain telemetric contact with the implantable device, which is further limited by its limited battery capacity to short communication sessions at low power or slow speeds.
In the monitoring of long term ECGs for features indicating intermittent heart irregularities, syncopal events and the like, minimally invasive monitors like the Reveal (TM) electrocardiogram event recorder manufactured by Medtronic, Inc. have proven to be useful, and now appear to be accepted by a segment of the medical community for use in diagnosing patient problems like fainting. However, particularly when the device employs automatic arrhythmia detection triggers to activate the storage of a segment of the ECG, the presence of noise in the ECG signal channel may trigger activations of recordings inappropriately, causing the device memory to become full of unwanted or redundant portions of the cardiac electrogram which may be of little to no use in diagnosing the patient condition Also, such noise when present may show up as a noisy signal in the recorded data, making interpretation of the signal and, diagnosis based on such interpretation, difficult.
It is felt that if the reconstituted electrogram read out for the physician had markers for identifying the type of noise present or the particular kind of trigger that caused the electrogram segment to be recorded, then the interpretation of even a noisy ECG would be easier and more accurate.
However, in order to store such information regarding the kind of noise or the nature of the auto-trigger or both, it might be thought that a separate memory or at least a separate location in memory from the ECG storage area would be required, along with a way to identify which marker was associated with any given segment of ECG data storage.
An additional complexity can be found in the limitation on the nature of the data available to store electrogram data samples, especially when, for one example, the sample rate produces more electrogram features than are stored via a lossy data compression technique in long term monitoring devices, a process relied upon to save memory and achieve sufficient data storage capacity to assist the physician in evaluating a long term ECG.
Accordingly we have developed a method and apparatus for identifying information available to an implantable medical device in an ECG data storage memory area which accommodates of the nature of the data compression and data communication requirements of the medical device.
Monitoring can be done using implantable pulse generators such as pacemakers and other heart stimulating devices or devices with leads in the heart for capturing physiologic parameters, including the ECG. However, the expense and risk from implanting a pacemaker or changing out one without these functions is something both patients and physicians would prefer to avoid. Such devices, in addition to performing therapeutic operations, may monitor and transmit cardiac electrical signals (e.g., intracardiac electrograms) to external diagnostic devices typically with leads fixed in the patient's heart, to observe electrical activity of a heart. It is common for implanted cardiac stimulation devices to send intracardiac ECG signals to a monitoring device, such as an external programmer, to allow a user to analyze the interaction between the heart and the implanted device. Often the user can designate that the communication from the implantable device to the programmer include a transmission of codes which signal the occurrence of a cardiac event such as the delivery of a stimulation pulse or a spontaneous cardiac depolarization.
For example, U.S. Pat. No. 4,223,678, (incorporated herein by this reference in its entirety) entitled “Arrhythmia Recorder for Use with an Implantable Defibrillator”, issued to Langer et al. on Sep. 23, 1980, discloses an arrhythmia record/playback component within an implantable defibrillator. ECG data is converted from analog to digital (A/D) form and stored in a first-in, first-out memory. When the defibrillator detects an arrhythmia event, it disables the memory so that no further ECG data is recorded in the memory until a command is received from an external monitoring device. This command requests the implantable defibrillator to transmit the stored ECG data to the monitoring device via telemetry. Langer et al. in U.S. Pat. No. 4,407,288, (also incorporated by reference herein) entitled “Implantable Heart Stimulator and Stimulation Method”, issued Oct. 4, 1983, discloses a programmable, microprocessor based implantable defibrillator which senses and loads ECG data into a memory via a direct memory access operation. A processor analyzes this ECG data in the memory to detect the occurrence of an arrhythmia event afflicting a patient's heart. Upon such an event, the defibrillator may generate a therapy to terminate the arrhythmia event and store the ECG data sequence of the event, for transmission to an external monitoring device and later study. In normal circumstances, when no arrhythmia event is occurring, the defibrillator continuously overwrites the ECG data in the memory.
U.S. Pat. No. 4,556,063, (too, incorporated herein by this reference) entitled “Telemetry System for a Medical Device”, granted to D. L. Thompson et al, 1985, teaches a pulse interval telemetry system capable of transmitting analog data, such as sensed intracardiac electrogram signals, without converting analog data to a digital numeric value. The Thompson et al. telemetry system is capable of sequentially transmitting both digital and analog data, individually and serially, in either an analog or a digital format, to a remote receiver. The features and capabilities of these pacemaker/defibrillator devices is now well known, but the problems in long term monitoring for events and adequate recordation and interpretations of noisy excessively triggered records remain.
Other background includes an article in the December 1992 Vol. 15 edition of PACE (15:588), a feasibility study for implantable arrhythmia monitors and reported by Leitch et al. Subcutaneous, Bipolar “Pseudo-ECG” Recordings using an Implantable Monitoring System and at chaired poster presentation of the North American Society of Pacing and Electrophysiology (NASPE).
Further, a leadless implantable sensor for cardiac emergency warning was described in U.S. Pat. No 5,404,887 issued to Knowlan et al. which detects heart events through impedance measurement sensed using a coil. See also Yomtov et al, U.S. Pat. No. 5,313,953 (incorporated herein by this reference) which describes (in FIG. 26) a large but leadless implant.
With sufficient hardware and connections to the body, numerous other physiologic parameters may be sensed as is pointed out in U.S. Pat. No. 5,464,434 issued to Alt and U.S. Pat. No. 5,464,431 issued to Adams et al. (both also incorporated herein by this reference).
Nevertheless there is still a need to indicate what kind of noise is present in a particular ECG segment and to do so in an efficient manner within the constraints imposed by the limitations of inexpensive devices with limited communications capacity, limited battery strength and limited time to communicate, and limited memory capacity, especially where the signal stored in memory may be complicated with data compression.
Of course, there is substantially more data that would be useful to capture along with the noise, including other physiologic condition sensor data, apparent R-waves which may be used for the arrhythmia triggers, indications of losing contact with the body by the electrodes, detecting pacing pulses, defibrillation pulses, low battery and other internal to the device conditions and so on. All such data may be substituted for interchangeably in various preferred embodiments of the present invention.
Presently without some indication of what kind of interfering or influencing signals are present, especially in situations where the ECG is reconstructed from a compressed electrogram storage, it is difficult to interpret the reconstructed ECG display.
The kinds of influences include ElectroMyoGraphic (EMG) noise from muscle activity, artifact noise from electrode motion within the body, loss or change in the electrode/body contact, pacemaker pulses, defibrillator pulses, and Electro-Magnetic Interference (EMI), which can be of a wide variety of types from different sources.
In addition it may be difficult to interpret the reason an automatic arrhythmia detection process chose to record the ECG segment being studied in the presence of such interfering signals, or in the absence of some interfering signals where filtering techniques or other anti-noise responses have eliminated much of the noise signal itself. It is not easy to answer the question: “Was the trigger set off by the noise, or not?”, and yet it is important for a valid diagnosis, to be able to do so. Too, with subcutaneous, or far field electrodes, ECG signal amplitude may vary greatly with mere change in patient posture; therefore knowing whether the recorded signal is a real arrhythmia or an artifact of poor detection performance is very difficult without real time information about the conditions present when the ECG sample was taken.
One final complication may arise where an ECG segment can be recorded by a patient controlled activation device. There needs to be room enough to recognize this extra signal and reason for recording the electrogram.